The present invention relates to an apparatus for detecting an in-cylinder pressure of an engine, and, more particularly, it relates to an apparatus for detecting an in-cylinder pressure more accurately by correcting a drift of the in-cylinder pressure.
Conventionally, an in-cylinder pressure sensor is provided in a cylinder of an engine so as to detect a pressure within the cylinder (this pressure will be hereinafter referred to as an in-cylinder pressure). The in-cylinder pressure detected by such a sensor is used in various control for the engine.
A sensor using a piezo-electric element is known as an in-cylinder pressure sensor. This sensor detects the rate of change of the in-cylinder pressure. As shown in FIG. 31, the change rate of the in-cylinder pressure detected by an in-cylinder pressure sensor 500 is typically integrated by an integrator circuit 501. An output of the integrator circuit 501 is used as the in-cylinder pressure.
In general, when the piezo-electric element is used, there are hysteresis characteristics in a relationship between a change in the actual in-cylinder pressure and an output of the in-cylinder pressure sensor. Furthermore, the output of the in-cylinder pressure sensor increases as the temperature of the piezo-electric element increases. When such an in-cylinder pressure sensor is mounted on the engine, variations occur in the output of the in-cylinder pressure sensor due to the heat generation from the engine. As a result, “deviation” (or drift) appears in the waveform of the in-cylinder pressure generated by the integrator circuit as shown in FIG. 32.
If such a drift appears, it may be difficult to detect the in-cylinder pressure with accuracy. The output of the in-cylinder pressure sensor is typically converted from analog to digital (A/D conversion) for subsequent computer processes. If a drift component is contained in the output of the in-cylinder pressure sensor, a correlation may be lost between the analog value of the output of the in-cylinder pressure sensor and the digital value obtained by the A/D conversion.
The Japanese Patent Publication No. H07-280686 discloses a technique for resetting an integrator circuit so as to correct such a drift. Referring to FIG. 33, a switching element 512 is closed at a predetermined timing in each combustion cycle of the engine. When the switching element is closed, the potential difference (voltage) across a capacitor 513 becomes zero. An output of an operational amplifier 514 is reset to a reference value. In response to such a resetting operation, a drift is removed.
FIG. 34 shows the waveform of the in-cylinder pressure generated by the integrator circuit when the above-described resetting operation is performed. The resetting operation is carried out at time t1, t2, t3, t4 and t5. It is seen that a waveform 515 caused by such a resetting operation is superposed on the waveform of the in-cylinder pressure. As a result, discontinuous frequency characteristics appear in the in-cylinder pressure waveform around the resetting operation. Due to such discontinuous frequency characteristics, an undesired frequency component may be introduced in subsequent computer processes that use the in-cylinder pressure. This may reduce the accuracy of control of the engine. Also, even if such a resetting operation is performed, a drift increases between resetting operations (that is, during one combustion cycle).
Thus, there is a need for an apparatus and method for preventing a drift from appearing in the in-cylinder pressure without performing such a resetting operation.